A polymer electrolyte fuel cell (PEFC) is a clean energy device that generates only water by causing an oxidation reaction of hydrogen on the anode and a reduction reaction of oxygen on the cathode. Polymer electrolyte fuel cells including platinum (Pt) as a cathode catalyst are known. Catalysts containing platinum, which is a noble metal, have advantages that they have high catalytic activity and electrical conductivity, and that they are not susceptible to corrosion and poisoning by the circumstances of the surrounding environment or substances present in the surrounding environment.
Meanwhile, there is a problem that the platinum resource is scarce and costly, and various studies are under way to improve the utilization efficiency and durability of platinum to reduce the used amount thereof. As one of such studies, a platinum core-shell catalyst obtained by covering a dissimilar metal with platinum is attracting attention. The platinum core-shell catalyst was devised focusing on the fact that only the platinum atoms exposed on the outermost layer of the catalyst particles exhibit catalytic activity, and has a structure in which fine particles of a dissimilar metal (core) covered with a platinum atomic layer (shell) are highly dispersed and supported on a support such as carbon.
As one of core metals of the platinum core-shell catalyst, palladium (Pd) is known. Non-Patent Documents 1 and 2 disclose that use of Pd as a core metal increases the oxygen reduction reaction (ORR) activity in a PEFC. Since the lattice constant of Pd (0.38898 nm) is smaller than that of Pt (0.39231 nm), small compressive stress is generated in the Pt shell provided on the Pd core. It is believed that this compressive stress realizes a situation where the oxygen reduction reaction is likely to proceed on the Pt shell surface to increase the ORR activity.
The Pd core-Pt shell structured catalyst has a problem in its durability since the standard redox potential of Pd (0.92 V vs. NHE) is lower than that of Pt (1.19 V vs. NHE), although the ORR activity of the catalyst is improved as described above. In Non-Patent Document 3, it is reported that in the PEFC using a carbon supported Pd core-Pt shell structured cathode catalyst (hereinafter, sometimes referred to as a Pt/Pd/C catalyst), the Pd core is partially oxidized and dissolved by power generation and that metallic Pd is re-precipitated in the solid polymer electrolyte membrane, forming a Pd band.
Although the present inventors have already found that the oxidative dissolution of the Pd core are problem from the viewpoint of durability of the catalyst, the oxidative dissolution of the Pd core causes changes in the particle size and morphology of the Pt/Pd/C catalyst, which enhances the ORR activity. Patent Document 1 discloses that an accelerated durability test (ADT) enhances the ORR activity of the Pt/Pd/C catalyst. Patent Document 1 also discloses that the ORR activity of the Pt/Pd/C catalyst can be enhanced by repeatedly applying a potential higher than the onset potential for Pt oxide reduction and a potential lower than the onset potential for formation of Pt oxide to the Pt/Pd/C catalyst.
In the specific potential application method disclosed in Patent Document 1, a platinum core-shell catalyst is dispersed in an acidic solution containing protons, and stirred under oxygen supply with coexistence of a metal having a lower redox potential than the onset potential for formation of Pt oxide. This method is an entirely novel technique and has a certain effect, but further enhancement in the ORR activity is required.
Meanwhile, Patent Document 2 discloses a platinum alloy catalyst and a method for manufacturing the platinum alloy catalyst. The manufacturing method disclosed in Patent Document 2 includes the steps of pressurizing and heating a mixed solution, which is obtained by dispersing an organometallic complex of platinum and a metal chloride in an organic solvent and then adding a reducing agent thereto, to synthesize platinum alloy nanoparticles having a size of 2 nm or less, and heating (annealing) the platinum alloy nanoparticles at a temperature of 300° C. or more and 1000° C. or less in a vacuum to adjust the diameter to 2 nm or more and 100 nm or less. It is believed that in the invention of Patent Document 2, platinum alloy particles having a specific crystal form can be obtained by synthesizing platinum alloy nanoparticles and then heating (annealing) the platinum alloy nanoparticles to adjust the particle size of the platinum alloy nanoparticles to 2 nm or more and 100 nm or less.